Vision in dim light: highlights and challenges.

On amoonless night or in the depths of the sea, where light levels aremany orders of magnitude dimmer than sunlight, animals rely on their visual systems to orient andnavigate, to find foodandmates and toavoidpredators. To seewell at such low light levels is far from trivial. The paucity of light means that visual signals generated in the light-sensitive photoreceptors of the retina can easily be drowned in neural noise. Despite this, research over the past 15 years has revealed that nocturnal and deep-sea animals—even very small animals like insects with tiny eyes and brains—can have formidable visual abilities in dim light. The latest research in the field is nowbeginning to reveal how this visual performance is possible, and inparticular which optical and neural strategies have evolved that permit reliable vision in dim light. This flurry of research is rapidly changing our understanding of both the limitations and the capabilities of animals active in very dim light. For instance, while the long-held viewwas that night vision allows only an impoverished, noisy and monochrome view of the world, we now know that many nocturnal animals see the world more or less in the same manner as their day-active relatives. Many are able to see colour, to use optic flow cues to control flight, and to navigate using learned visual landmarks and celestial cues such as polarized light. Much of our appreciation of the richness of the visual world seen by nocturnal animals has derived primarily from behavioural, anatomical and optical studies. More recently, enormous advances have also been made in understanding the neural basis of this performance in both single cells and circuits of cells from both nocturnal vertebrates (notably mice) and nocturnal invertebrates (notably insects). These studies indicate that the remarkable behavioural performance of these animals in dim light can only partially be explained by what we currently know of the performance of the underlying visual cells. We are thus now at an important point in the field where this gap is closing. It is thus particularly timely that this special issue brings together a unique combination of recent research on deep-sea and nocturnal animals and moreover from a wide spectrum of scientific disciplines, from ecology, evolution and quantitative visual behaviour to cellular electrophysiology, mathematical modelling and molecular biology. This landmark collection of papers is the first to exclusively address the topic of comparative vision in dim light.